Coral reefs have been a hub for human activity for years and really, why wouldn’t they be? They are filled with glorious colors, animals ranging from unique corals to venomous invertebrates, and, of course, a variety of fishes which are vital sources of protein and income to many island communities. These fish-laden reefs have become a focus in conservation due to the increased fishing pressure humans have had on them. There are many coral reefs that are located in remote areas, several hundred kilometers away from human populations. These coral reefs have been used as a baseline to determine the effect humans have on reef ecosystems. It has been shown that remote coral reefs typically have about three or four times the fish biomass as compared to human-populated island reefs.

Williams et al. decided to take this information and build off of it. Are humans the sole contributor or are there other factors that account for differences in fish biomass? A wide range of data was collected to determine the impact that not only humans, but other oceanographic gradients have on coral reef ecosystems.

Fig 1. Locations of reef areas (i.e. islands and atolls) included in this study.

Determining the baseline

To compare a human populated coral reef status to that of a “pristine” coral reef (reef void of human impact) researchers accumulated over 2,000 hours underwater! The researchers spent a great deal of time on research vessels (1-3 months at any given time), diving and conducting visual surveys on the reef fish populations. They monitored human-populated island reefs such as the island of Maui in Hawaii, as well as some of the most isolated coral reefs in the world such as the Rose Atoll (Fig. 1). Fishes were counted and size estimations were made underwater. This information was then coupled to the satellite-derived information taken on the same reefs. Environmental conditions such as reef location, surface temperature, wave energy, and oceanic productivity were used as variables to generate a model separating the effects that humans have verse the effects large-scale environmental factors have on reef fish abundance.

How humans impact coral reefs and why we should care

The resulting message of this study is pretty sobering. The results show anywhere from 20% to 78% depletion on human populated reefs compared to isolated “pristine” reefs. The fish most susceptible to human impacts were piscivores and higher trophic level fish species, such as the grouper (Fig. 2). While the majority of reefs with low fish abundance were centered around human activity, certain human-populated reefs were seen with a higher fish biomass than certain isolated reefs. Coral reefs situated close to the equator, where nutrient rich water is replenished via upwelling, were typically higher in fish biomass, regardless of human activity.

This study not only highlights how even small scale human activity can have an effect on coral reef ecosystems, but it also highlights that not all reefs are the same. In other words, to compare coral reefs across the world is like comparing apples to oranges. The capacity of coral reefs to support large fish populations varies among areas and is not only determined by human influence, but also by oceanic environmental factors. This emphasizes that there is no single standard for what a healthy reef should look like. That being said, the consistent decline in fish biomass on even low human populated reefs is continuing, and in order to maintain coral reefs in their natural state, management strategies still need to be put in place.

Do you think humans have a significant impact on reef fish? If so, what do you do personally to help conserve the reef fish species? Comment below and let us know!

Fig 2. Visualization of smoothers for (a) local humans per area of forereef (HUM)

For my fisheries and aquatic science PhD I am working on how to tank raise urchins and transplant them onto reefs across the Florida Keys in order to help reverse the phase shift from algae dominated back to coral dominated.